"Mark as Answer" disappears after a thread has been exported and imported as a LAR

Description

1. post thread with 'Mark as a Question' checked.
2. reply to the thread and click 'Mark as an answer'
3. go to the category main page and you will see the status as 'Resolved'
4. go back to the answer thread and click 'Unmark' next to the answer
5. It will still display the status as 'Resolved'